Measurement of charm fragmentation ratios and fractionsin photoproduction at HERA

Abstract.

The production of $D^{\ast + }$, D⁰, D ⁺ , D _s ⁺ and $\Lambda_c^ +$ charm hadrons and their antiparticles in e p scattering at HERA was measured with the ZEUS detector using an integrated luminosity of $79 \rm {pb}^{-1}$. The measurement has been performed in the photoproduction regime with the exchanged-photon virtuality $Q^2 < 1 \rm {GeV}^2$ and for photon-proton centre-of-mass energies in the range $130 < W < 300 \rm {GeV}$. The charm hadrons were reconstructed in the range of transverse momentum $p_T(D,\Lambda_c) > 3.8 \rm {GeV}$ and pseudorapidity $\vert\eta(D,\Lambda_c)\vert < 1.6$. The production cross sections were used to determine the ratio of neutral and charged D-meson production rates, $\smash{R_{u/d}}$, the strangeness-suppression factor, $\gamma_s$, and the fraction of charged D mesons produced in a vector state, $\smash{P^d_{\rm v}}$. The measured $\smash{R_{u/d}}$ and $\smash{\gamma_s}$ values agree with those obtained in deep inelastic scattering and in e ⁺ e^- annihilations. The measured $\smash{P^d_{\rm v}}$ value is smaller than, but consistent with, the previous measurements. The fractions of c quarks hadronising as a particular charm hadron, $f(c \rightarrow D, \Lambda_c)$, were derived in the given kinematic range. The measured open-charm fragmentation fractions are consistent with previous results, although the measured $f(c\rightarrow D^{* + })$ is smaller and $f(c\rightarrow \Lambda_c^ + )$ is larger than those obtained in e ⁺ e^- annihilations. These results generally support the hypothesis that fragmentation proceeds independently of the hard sub-process.